[callmeal]

>I suspect that Amazon does not want to put additional cognitive load on the purchaser right at the most critical point of the conversion funnel.

They can do it off the side, like how you can select an amazon locker to ship to (but still have your default mailing address set to something else). They could easily use the same flow: just add a "pick your shipper" section similar to how some small time retailers too it and have separate pricing for each (if you're not on prime).